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for the zonally averaged circulation, but the longitudinally and time-varying com-
ponents as well.
Nevertheless, to introduce the study of the general circulation, it proves useful
to isolate those processes that maintain the zonal-mean flow (i.e., the flow aver-
aged around latitude circles). This approach follows naturally from the linear wave
studies of previous chapters in which flow fields were split into zonal-mean and
longitudinally dependent eddy components. In this chapter, however, we concen-
trate not on the development and motion of the eddies, but on the influence of
the eddies on the structure of the zonal-mean circulation. Focusing on the zonal
mean allows us to isolate those features of the circulation that are not dependent
on continentality, and should thus be common to all thermally driven rotating fluid
systems. In particular, we discuss the angular momentum and energy budgets of
the zonally averaged flow. We also show that the mean meridional circulation
(i.e., the circulation consisting of the zonal-mean vertical and meridional veloc-
ity components) satisfies a diagnostic equation analogous to the omega equation
of Section 6.4.1, but with the forcing determined by the distributions of diabatic
heating and eddy heat and momentum fluxes.
Following our discussion of the zonal-mean circulation we consider the longi-
tudinally varying time averaged circulation. In this chapter the primary emphasis
is on extratropical aspects of the circulation; these can be discussed within the
framework of quasi-geostrophic theory. The general circulation of the tropics is
considered in Chapter 11.
10.1
THE NATURE OF THE PROBLEM
Theoretical speculation on the nature of the general circulation has quite a long
history. Perhaps the most important early work on the subject was that of the 18th-
century Englishman, George Hadley. Hadley, in seeking a cause for the trade wind
circulation, realized that this circulation must be a form of thermal convection
driven by the difference in solar heating between equatorial and polar regions. He
visualized the general circulation as consisting of a zonally symmetric overturning
in which the heated equatorial air rises and flows poleward where it cools, sinks, and
flows equatorward again. At the same time, the Coriolis force deflects the poleward
moving air at the upper levels to the east, and the equatorward moving air near the
surface to the west. The latter is, of course, consistent with the observed surface
winds in the trade wind zone, which are northeasterly in the Northern Hemisphere
and southeasterly in the Southern Hemisphere. This type of circulation is now
called a
Hadley circulation
or Hadley cell.
Although a circulation consisting of Hadley cells extending from equator to pole
in each hemisphere is mathematically possible in the sense that such a circulation
would not violate the laws of physics, the observed Hadley circulation is confined to
